1. Field of the Invention
The present invention relates to methods and apparatus for monitoring motion of objects and more particularly concerns an internal laser interferometer system having increased sensitivity to motion of an object to be monitored and decreased sensitivity to effects of beat frequencies within the laser.
2. Description of Related Art
Testing, calibration and monitoring of vibratory systems such as ultrasonic bonders, acceleration testers, speakers and the like requires high precision measurement of motion, including displacements and velocities. A system capable of making measurements with the high precision required is shown in U.S. Pat. No. 3,644,042, entitled "Laser System For Monitoring The Motion Of Object". This patent discloses what is basically an internal laser interferometer in which motion of an object is monitored by directing a laser beam from a laser oscillator toward the object to be monitored and receiving energy retroreflected by the object back into the laser cavity. Depending upon the phase relation between the projected and reflected energy, oscillation intensity within the laser cavity is increased or decreased by the reflected incoming energy. When the object is at a distance from the laser cavity, that is an even integral multiple of one-half wavelength of the projected beam so that the retroreflected energy is in phase with cavity oscillations, the oscillations in the cavity are enhanced to a maximum extent. When the distance is an odd integral multiple of one-half wavelength, and reflected energy is of opposite phase, there is maximum destructive interference. As the object moves toward or away from the laser cavity, intensity of the oscillations within the cavity varies through successive maxima and minima, at successive one-half wavelength positions of the object, so that a count of these maxima and minima provides an indication of the displacement of the object, and the number of maxima and minima per unit of time indicates velocity. The maxima and minima are detected by transmitting a portion of the oscillations within the cavity out through the back reflector of the cavity to a suitable circuit including a detector sensitive to energy at the frequency of the laser beam.
The patent describes a Helium Neon laser in which oscillations at both 0.6328 and 3.39 micrometers are produced. The longer wavelength is transmitted to and reflected from the object to be monitored and re-enters the cavity to modulate the shorter visible wavelength (0.6328 micrometers) by virtue of inherent "competition effect" between the energy waves produced by the laser at the two wavelengths. Intensity of the shorter wavelength is detected to provide the desired output.
Equipment based upon principles of the system disclosed in the U.S. Pat. No. 3,644,042, is manufactured and sold by the assignee of the present invention as the "Hughes Model 2100 Ultrasonic Monitor". This equipment has been employed widely for monitoring of ultrasonic wire bonders and has enjoyed significant success. The equipment, although highly precise in its measurement, has a relatively low sensitivity that requires it to be positioned close to the object being monitored. Commonly, this ultrasonic motion monitor must be placed within one inch of the object being monitored in order to provide a variation of amplitude of the oscillations within the laser strong enough to be detected. Even so, output signal levels have measured amplitude variations of millivolts, a signal low enough to occasionally be in the noise level of the instrument. Furthermore, the system tends to generate its own interference fringe patterns, particularly during its relatively long warm-up time due to mode scanning. The prior system as described in U.S. Pat. No. 3,644,042, and as embodied in the Hughes Model 2100 Ultrasonic Monitor employs a laser having three longitudinal modes of oscillation at the 0.6328 micrometer output, which give rise to a beat frequency of about 60 kilohertz. This beat frequency is within the common frequency range (of about 15 to 100 kilohertz) of many ultrasonic bonders and prevents use of the instrument with bonders during a required warm-up time (commonly about 20 minutes) of the system. These beat frequencies change both in frequency and intensity as the cavity length of the laser changes during warm-up of the structure and provide spurious interference fringes that significantly interfere with the fringes or intensity beat variations caused by light reflected from the object being monitored. Not only do these spurious fringes generated by beat frequencies cause problems during warm up, thus requiring the long warm-up time, but they cause significant problems when the system is used in an environment that may experience temperature changes.
Accordingly, it is an object of the present invention to provide a motion monitoring system that has increased sensitivity and in which problems due to beat frequencies are minimized.